As used herein, the term battery is intended to define the source of direct-current electrical energy of a motor vehicle, comprising recharging and stabilizing circuits in addition to the actual battery. The term car radio relates not only to a radio receiver but also to any electronic apparatus, supplied by the battery, for reproducing sound or for other uses.
As is known when the engine of the motor-vehicle is started, and also during normal operation of the motor vehicle, the direct voltage generated by the battery is subject to interference, such as reductions in level and pulsed positive and negative transients, which may cause malfunctioning or even breakdown of electronic devices connected to the battery. In the case of a car radio, a reduction in level below a minimum permitted voltage may cause the apparatus to switch off, and pulsed transients may cause annoying noises to be emitted by the loud speakers.
It is also known that incorrect connection of the battery (reversal of polarity) can cause serious damage to the apparatus supplied by the battery.
To limit interference, a filter is usually used in the circuit connecting the car radio to the battery. To prevent damage due to accidental reversal of the battery polarity, suitable protection means are used in the connection circuit. A typical connection circuit comprising a filter and a Protection device is shown in FIG. 1. The circuit, indicated by the reference number 10 in the drawing, is interposed between a battery 11 and a car radio 12 and is normally located physically in the container that houses the car radio. The circuit is constituted by an LC filter and by a power diode. The LC filter in this example is a two-pole, low-pass filter and is formed by an inductor L in series, that is, connected between the positive pole of the battery 11 and a supply terminal of the car radio 12, and by a capacitor C in parallel with the car radio supply terminals. The diode, indicated Dp, is also connected in parallel with the car radio supply terminals, more precisely, for reverse conduction, that is, with its anode connected to the terminal of the circuit that is to be connected to the negative pole of the battery, so that the diode conducts only when the battery is connected with reversed polarity. A fuse 13, which intervenes and interrupts the supply in the event of overloading, is normally provided between the positive pole of the battery 12 and the connection circuit 10.
It is known that, in order to increase the output power of the car radio or of other apparatus supplied by the motor-vehicle battery, it is possible to use a voltage raising circuit as described, for example, in Italian patent application No. MI99A000257 filed by the Applicant herein on Feb. 10th 1999. When necessary, this circuit is connected between the filtering and protection circuit 10 and the supply terminals of the apparatus.
The known filtering and protection circuit 10 of FIG. 1 has some disadvantages, as a result of which it is not always reliable and effective in use. In particular, if the battery is connected with reversed polarity, the voltage drop in the diode is quite high, for example 2 V, so that, between the supply terminals of the car radio, there will be a negative voltage (-2 V), which may damage the apparatus before the fuse intervenes and interrupts the connection to the battery. Moreover, a negative transient in the battery voltage Vbatt is translated, at the output of the filter, into a voltage Vfil with oscillations of considerable amplitude that are also potentially damaging to the apparatus. For example, as is shown in the graph of FIG. 3, relating to an apparatus that absorbs a current of 2A, a transient of the battery voltage Vbatt which starts at 14 V, falls to 0 V and rises again to 18 V before returning to 14 V brings about a ripple of the filter output voltage Vfil, which goes from -0.7 V to 26 V.